6.26 Designated Initializers

Standard C90 requires the elements of an initializer to appear in a fixed
order, the same as the order of the elements in the array or structure
being initialized.

In ISO C99 you can give the elements in any order, specifying the array
indices or structure field names they apply to, and GNU C allows this as
an extension in C90 mode as well. This extension is not
implemented in GNU C++.

To specify an array index, write
`[index] =' before the element value. For example,

int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being
initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but
GCC still accepts is to write `[index]' before the element
value, with no `='.

To initialize a range of elements to the same value, write
`[first ... last] = value'. This is a GNU
extension. For example,

int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only once,
not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified
plus one.

In a structure initializer, specify the name of a field to initialize
with `.fieldname =' before the element value. For example,
given the following structure,

struct point { int x, y; };

the following initialization

struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

struct point p = { xvalue, yvalue };

Another syntax which has the same meaning, obsolete since GCC 2.5, is
`fieldname:', as shown here:

struct point p = { y: yvalue, x: xvalue };

The `[index]' or `.fieldname' is known as a
designator. You can also use a designator (or the obsolete colon
syntax) when initializing a union, to specify which element of the union
should be used. For example,

union foo { int i; double d; };
union foo f = { .d = 4 };

will convert 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
would store it into the union as the integer i, since it is
an integer. (See Cast to Union.)

You can combine this technique of naming elements with ordinary C
initialization of successive elements. Each initializer element that
does not have a designator applies to the next consecutive element of the
array or structure. For example,

int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

int a[6] = { 0, v1, v2, 0, v4, 0 };

Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:

You can also write a series of `.fieldname' and
`[index]' designators before an `=' to specify a
nested subobject to initialize; the list is taken relative to the
subobject corresponding to the closest surrounding brace pair. For
example, with the `struct point' declaration above:

struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from
the last initialization. If any such overridden initialization has
side-effect, it is unspecified whether the side-effect happens or not.
Currently, GCC will discard them and issue a warning.